The relationship between nocturnal sleep and daytime behavior in children is not well understood. Sleep-related breathing disorders are a frequent cause of disrupted sleep in children and may cause daytime behavioral problems, but such consequences have not been well characterized and currently most children with such sleep disorders remain undiagnosed. Sleep-disordered breathing (SDB) is especially common in children with large tonsils, and effective treatment can be accomplished, once a diagnosis is established, by removal of the tonsils and adenoids in a procedure known as adenotonsillectomy. Descriptive case series have suggested that unlike SDB in adults, which often causes excessive daytime sleepiness, SDB in children may be associated with problems in behavior, thinking, learning or emotional control. The purpose of the current research is to better define the relationship between SDB and these types of daytime problems especially inattention, hyperactivity, and other disruptive behaviors and to determine whether SDB may actually cause these behaviors. Children who are 5 through 12 years of age and who are scheduled for either adenotonsillectomy or hernia repair (control group) will be recruited to participate in this study. A few weeks before and one year after their surgery children will participate in a series of tests to evaluate their sleep, behavior, and thinking. The results of these studies will help determine how common behavioral problems are in children who are scheduled for adenotonsillectomies. The study will also help to determine whether SDB is associated with specific behavioral or learning problems in children and whether adenotonsillectomy is an effective treatment both for SDB and for associated behavior problems. In addition, valuable information will be obtained on whether SDB causes inattention, hyperactivity, and related behaviors in some children. The results of this study may lead to better clinical guidelines for the diagnosis and treatment of childhood SDB and may help to reduce the burden of important behavioral problems for some children and their families.